1. Field of the Invention
This invention relates generally to apparatus, such as, a video tape recorder (VTR) of the so-called "helical-scan type", in which video or other information signals are recorded in successive parallel tracks which are skewed or extend obliquely on a magnetic tape, and more particularly is directed to an improved system by which a magnetic head or other transducer in such apparatus is made to accurately scan the track or tracks in which the video or other information signals are recorded.
2. Description of the Prior Art
In a conventional helical-scan VTR, the magnetic tape extends helically about at least a portion of the periphery of a guide drum and is adapted to be moved or advanced in the longitudinal direction of the tape while at least a portion of the guide drum is rotated, and the transducer or magnetic head is mounted on a rotated portion of the guide drum so as to rotate with the latter and thereby repeatedly scan across the tape in a path at an angle to the longitudinal direction of the tape. During recording operation of the VTR, the angle between the scanning path, and hence each record track, and the longitudinal direction of the tape is dependent on the rotational speed of the rotary head and also the speed at which the magnetic tape is longitudinally advanced. Accordingly, if the rotational speed of the rotary head and/or the speed and direction of advancement of the magnetic tape are not the same during the reproducing operation as during the recording operation, then the scanning path of the magnetic head during reproducing will not precisely follow or coincide with a record track on the tape during each movement of the head across the tape and, accordingly, the recorded video or other information signals may not be correctly or accurately reproduced. For example, if the magnetic tape is at rest during a reproducing operation, as in the still-motion mode of operation, the angle of the scanning path of the rotary magnetic head relative to the longitudinal direction of the tape may be sufficiently different from the angle at which the record tracks are skewed that the magnetic head scans portions of two adjacent record tracks during initial and concluding portions of its scanning movement across the tape. Even when the speed of advancement of the tape is the same during the reproducing and recording operations, the magnetic head may not correctly scan each of the successive record tracks as a result of stretching or other deformation of the tape following the recording operation.
Various arrangements have been proposed for maintaining correct tracking or scanning of the record tracks by the rotary head. In the most desirable of these known arrangements, means are provided for deflecting the head in a direction normal to the plane of its rotation, that is, in a direction which is transverse in respect to the direction along each of the record tracks, and the amount of such deflection is electrically controlled during the movement of the head along each track so as to achieve accurate scanning of the latter. In published Japanese Patent Application No. 9919/1974 (Applicant: Matsushita Electric Industrial Company, Limited), it is generally disclosed to control the amount of deflection of the head in the direction normal to its plane of rotation in dependence on the difference between the speeds of advancement of the tape during the recording and reproducing operations so as to theoretically make possible the correct reproduction of the recorded video signals in the still-motion mode in which the tape is at rest, in the slow-motion mode in which the speed of advancement of the tape is, for example, 1/4 or 1/2 the tape speed for the recording operation, and in the quick-motion mode in which the speed of advancement of the tape is substantially greater than, for example, 2 times the tape speed, for the recording operation. Further, in Published Japanese Patent Application No. 117,106/1977 (Applicant: Ampex Corporation), it is disclosed to detect the amount of deflection of the head required for accurately scanning a record track thereby. When the detected deflection approaches the physical limit of that possible with the bi-morph leaf or other transducer deflecting means supporting the head, the electrical drive signal for the bi-morph leaf is controlled to cause the head to scan the next adjacent record track, that is, to effect a so-called "track jump". By reason of such "track jump" it is theoretically possible to effect correct slow-motion reproduction and also reverse-motion reproduction in which, for example, the tape is advanced longitudinally at the same speed as for the recording operation, but in the reverse or opposite direction.
However, in the non-normal reproduction modes, for example, the still-motion, slow-motion and reverse-motion modes, troubles arise by reason of time base variations of the reproduced signals. Such time base variations are caused mainly by the distance, measured in the direction along the record tracks, provided between the initial ends of the adjacent record tracks for achieving so-called H-alignment, and by the change occurring in the relative speed of the tape and magnetic head due to change-over of the speed of advancement of the tape. Time base variations are further caused by unavoidable fluctuations in the rotational speed of the magnetic head and in the speed of advancement of the tape during the reproducing operation. Of course, any fluctuations in the rotational speed of the magnetic head and the speed of advancement of the tape occurring during the recording operation are included in the recorded signals, and carried over into the reproduced signals to give rise to corresponding time base variations in the latter.
The above described time base variations can, for the most part, be corrected by means of existing time base correctors (TBC). However, the known time base correctors are generally not suitable for correcting time base errors of the magnitude encountered with track jumps by the magnetic head. For example, an excessive time base variation which cannot be corrected by existing time base correctors may be encountered in the case of a VTR with track-jumping capability when the speed of advancement of the tape is changed during the scanning of a particular record track or when the record track is severely deformed.
It has also been proposed, for example, as disclosed in Published Japanese Patent Application No. 84617/1974 (Applicant: Nippon Hoso Kyokai), to avoid mistracking due to deformation of the record track or the like by monitoring the position of the head relative to the record track during the scanning of the latter through reproduction of the recorded video or other information signals while a small oscillatory motion or dither of constant amplitude and period is imparted to the head via its supporting element or arm. The oscillation of the head introduces deviations in the envelope of the reproduced information signals obtained when more or less scanning the record track. Such deviations take the form of an amplitude modulation of the envelope of the reproduced signals, with the change in magnitude of the envelope being representative of the amount of traverse displacement of the head from the optimum position with respect to the track, and with the direction of traverse displacement of the head from the optimum transducing position being represented by the phase of the envelope amplitude modulation at the fundamental frequency of the oscillatory motion or dither. To obtain the head position information, the modulated RF envelope signal reproduced by the head is applied to an amplitude modulation envelope detector which recovers the dither signal fundamental and its sidebands, whereupon the output of the envelope detector is applied to a synchronous modulation detector which detects the amplitude and polarity of the output of the envelope detector with reference to the original or constant dither or oscillation signal by which the head is made to oscillate transversely. The synchronous amplitude modulation detector provides a tracking error signal which is added to the dither oscillation signal to provide the drive signal applied to the transducer deflecting means, such as, a bi-morph leaf, mounting the head for effecting the movements or deflections of the head in a direction transverse to the direction along each of the record tracks. Generally, the amplitude of the tracking error signal is proportional to the transverse distance from the null position of the oscillated head to the track center, while the polarity of the tracking error signal is indicative of the direction of such displacement of the null position from the track center. Therefore, the tracking error signal, when added to the dither or oscillation signal, tends to align the null position of the head with the center of the track. However, the frequency of vibration of the bi-morph leaf supporting the magnetic head is mechanically restricted to a certain range, whereby the response to the head to a tracking error is correspondingly restricted. When the VTR is capable of being operated in the so-called "jog" reproducing mode, or when the VTR is changed-over from its normal reproducing mode to a slow-motion reproducing mode, the speed of advancement of the tape is changed abruptly and correct tracking control of the magnetic head cannot be maintained by the previously described systems according to the prior art.